It is desirable to provide, in high-speed machinery, anti-friction rolling bearings, such as ball bearings or roller bearings, to reduce friction and permit high-rotational speeds with low-energy loss. However, in machines having shaft-rotating speeds on the order of 60,000 or 80,000 rpm, such bearings have a limited life on the order of 500 hours. Such a limited life is not acceptable in many commercial applications and, particularly, in applications such as the turbochargers for internal combustion engines.
Turbochargers for internal combustion engines are now widely used in conventional automobiles, and bearing systems, particularly as adapted for use in turbochargers, must be capable of mass-production methods and must be economical to manufacture and service. In addition, such bearing systems must have a long life and must not be subject to breakdown at relatively short intervals such as 500 hours. Since anti-friction bearings such as roller bearings and ball bearings are expensive and have too short an operating life for commercial use in automotive turbochargers, it has been conventional practice to provide sleeve bearings in such applications.
Conventional sleeve bearings have been found objectionable in these applications because they would not tolerate a practical degree of imbalance in their supported rotating parts and because they were incapable of damping resonant vibrations of the rotating parts due to such imbalance. Instead of rotating about a stationary center, unbalanced high-speed, rotating shafts have a tendency to whirl about in such a manner that their centers travel through a circular or modified circular path. Conventional bearing clearances limit such a whirling motion; but at speeds on the order of 80,000 rpm or more, metal-to-metal contact and rapid wear of the bearings occur as a result of oil film breakdown due to lubricating oil film dynamics and bearing loads.
A number of prior bearings have resulted from these problems. Among the patents disclosing bearings resulting from such problems, are prior U.S. Pat. Nos. 3,056,634; 3,096,126; 3,390,926; and 3,993,370. These patents are directed to bearings to provide stability at high speed with the ability to absorb vibration and shock load of the type experienced by bearing systems, for example, in turbochargers, and to be manufactured at low cost. The bearings of the patents above generally solved the stability problem by the use of a free-floating bushing between the rotating shaft and its stationary supporting member which was adapted to provide a film of lubrication between its inner surface and the rotating shaft and between its outer surface and the stationary supporting member. In these systems, the free-floating bushing was free to rotate, but at speeds substantially less than the rotating shaft and was free to move radially in response to imbalance experienced by the rotating shaft. The freedom of radial movement permitted the rotating shaft to rotate about its center of mass and the inner and outer oil films provided the necessary lubrication and cushion against vibration and shock loads. The problem of "oil whirl" where the pressures in the film of lubrication go to zero and the shaft contacts the bearing surface was solved.
The bearings of these patents also provided thrust-bearing surfaces as, for example, in U.S. Pat. No. 3,390,926 and employed the increased frictional drag at the outer bushing surface to reduce the rotational speed of the bushing to a fraction of the rotating shaft speed and passageways in the bearings themselves to direct oil from the outer surface to the inner surfaces and thrust-bearing surfaces.
U.S. Pat. No. 4,370,106 suggests a bearing system for a turbocharger rotor, including an anti-friction ball bearing at its compressor end and a sleeve bearing at its turbine end. This bearing system, however, includes a non-rotating sleeve that forms a non-rotating sleeve bearing at the turbine end of the rotor and an integral non-rotating support for the outer race of the anti-friction ball bearing at the compressor end of the rotor. The sleeve and bearing system is prevented from rotating by a square or out-of-round portion at the compressor end that is mechanically engaged with the turbocharger housing. Lubricant is provided between the stationary sleeve and the supporting housing to provide resilience and damping for eccentric motion of the rotor due to its imbalance. In this bearing system, however, the differential speed between the sleeve bearing and rotor is the full rotative speed of the rotor. Since the bearing losses are proportional to the square of the rotative speed, this system was a high loss compared with a floating bearing system. In addition, since the non-rotating sleeve must bear against the stationary housing with the full thrust load of the rotor. The imbalance in the rotor forces the non-rotating sleeve to move orbitally, causing the surfaces to be subject to fretting. Thus, a stationary solid film lubricant must be provided to lessen the fretting problem. However, the problem is still inherent with this non-rotating sleeve system and contributes to a reduced service life in the field.
Other patents directed to bearings intended to solve the stability problem are U.S. Pat. Nos. 3,043,636 and 3,811,741; but the bearings of these systems have substantial hydrodynamic losses associated with the bearings which reduce the efficiency of a turbocharger.